As is widely known, R—Fe—B based permanent magnets such as Nd—Fe—B based permanent magnets are produced from inexpensive materials, whose resources are abundant, and also have high magnetic characteristics, and thus they are used in various fields today. Against such a background, in R—Fe—B based permanent magnet production plants, magnets are produced in large amounts every day. However, with an increase in the amount of magnets produced, the amount of magnet scrap discharged as a defectively processed product or the like, magnet sludge discharged as cutting waste, grinding waste, or the like, etc., in the production process has also been increasing. In particular, with the weight and size reduction of information devices, the size of magnets used therein has also been reduced, leading to an increase in the proportion of processing allowance, and, as a result, the production yield tends to decrease year by year. Accordingly, how to recycle magnet scrap, magnet sludge, and the like discharged in the production process rather than discarding them is an important technical challenge for the future. The same also applies to how to recycle used magnets recovered from waste electrical appliances and the like.
An R—Fe—B based permanent magnet is generally produced through a process in which a plurality of raw material metals are mixed in a predetermined ratio and subjected to high-frequency heating in a vacuum melting furnace to give an alloy material of predetermined composition. Considering the case where magnet scrap, magnet sludge, used magnets, and the like are recycled for the production of a magnet, in terms of energy saving, cost reduction, etc., it would be ideal if they could be directly subjected to high-frequency heating in a vacuum melting furnace to give an alloy recycled material, but it has not been put into practice in reality. One of the reasons is that magnet scrap, magnet sludge, used magnets, and the like contain, in the magnet structure, carbon from an organic lubricant or the like used in the magnet production process, and thus, when an alloy recycled material is obtained therefrom and used to produce a magnet, carbon contained therein adversely affects the magnetic characteristics of the magnet. Accordingly, in order to recycle magnet scrap, magnet sludge, used magnets, and the like for the production of a magnet, it is preferable to remove carbon contained therein.
As a method for removing carbon from a carbon-containing R—Fe—B based permanent magnet alloy such as magnet scrap, magnet sludge, a used magnet, or the like, for example, Patent Document 1 proposes a method in which carbon contained in the alloy is reduced using calcium metal or calcium hydride as a reducing agent, thus converted into calcium carbide, and removed. However, this method has a problem in that because rare earth carbides are thermodynamically more stable than calcium carbide, a rare earth carbide is produced prior to calcium carbide, and thus a large amount of rare earth carbide is removed, resulting in a poor yield of rare earth element in the recycled alloy material. Patent Document 2 also discloses a method in which magnet scrap containing carbon in the form of powder is heat-treated in an oxygen atmosphere at a temperature of 700° C. to 1200° C. for 1 hour to 10 hours to cause oxidation and decarburization. However, this method has a problem in that a large amount of rare earth oxide is produced during oxidation and decarburization, and thus a large amount of calcium metal or the like is required as a reducing agent to reduce the produced rare earth oxide, resulting in high cost. Another problem is that calcium metal or the like used as a reducing agent is contained in the recycled alloy material as an impurity, resulting in adverse effects on the magnetic characteristics of the magnet. Therefore, according to heretofore proposed methods, it has not been possible to effectively remove carbon contained in magnet scrap, magnet sludge, a used magnet, or the like. Thus, in order to avoid a non-negligible increase in the amount of carbon contained in the magnet produced, in the actual situation, recycling of them for the production of a magnet is performed in a mode where they are fed to a vacuum melting furnace little by little, subjected to high-frequency heating together with a virgin alloy material, and thus used, or a mode where they are chemically recycled, recovered as a rare earth element, and thus used. However, these modes have problems in that, for example, even when an attempt is made to reduce the amount of virgin alloy material used for the purpose of cost reduction, the amount that can be reduced is naturally limited, and also in the case of chemical recycling, the impact of discharged liquid waste on the environment must be considered.